The present invention relates to a process for preparing a polyimide that has excellent thermal stability and processability in a molten state.
The present invention also relates to a polyimide composite material that has excellent high-temperature stability, chemical resistance and mechanical strength as well as outstanding processability.
A polyimide obtained by reacting a tetracarboxylic dianhydride and an aromatic diamine has excellent mechanical strength, dimensional stability, high temperature stability, flame resistance and electrical insulation properties. Such a polyimide has been used in electrical and electronic appliances, space and aeronautical instruments and transport machinery. It is expected that it will also be used in applications where high-temperature resistance is required.
A variety of polyimides that exhibit excellent properties have been developed. Some polyimides, however, have no distinct glass transition temperature, although they are excellent in high-temperature stability. As a result, when employed as molding materials, specific methods such as sinter molding must be used for processing. Other polyimides which are excellent in processability have low glass transition temperatures and are soluble in halogenated hydrocarbons, which renders these polyimides unsatisfactory for use in applications which require high-temperature stability and solvent resistance. Thus, these polyimides have numerous advantages and disadvantages.
The present inventors have previously discovered a polyimide that is excellent in mechanical strength, thermal characteristics, electrical properties and solvent resistance and exhibits high-temperature stability. The polyimide consists primarily of recurring units of the formula: ##STR1## wherein X represents a direct bond or a radical selected from the group consisting of a C.sub.1 -C.sub.10 divalent hydrocarbon radical, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical and a sulfonyl radical; Y.sub.1, Y.sub.2, Y.sub.3 and Y.sub.4 may be the same or different and represent a radical selected from the group consisting of a hydrogen atom, a lower alkyl radical, a lower alkoxy radical, a chlorine atom and a bromine atom; and R represents a tetravalent radical selected from the group consisting of an aliphatic radical having two or more carbon atoms, a cyclic aliphatic radical, a monocyclic aromatic radical, a fused polycyclic aromatic radical, and a polycyclic aromatic radical wherein the aromatic rings are linked together directly or via a bridged member. The polyimides have been disclosed in the following Japanese Laid-Open Patents:
Ohta et al; TOKKAISHO 61-143478 (1986).
Tamai et al; TOKKAISHO 62-68817 (1987), which corresponds to copending U.S. patent application Ser. No. 44,028, filed on June 30, 1986.
Ohta et al; TOKKAISHO 62-86021 (1987), which corresponds to copending U.S. patent application Ser. No. 44,028, filed on June 30 1986.
Ohta et al; TOKKAISHO 62-235381 (1987) and
Oikawa et al; TOKKAISHO 63-128025 (1988), which corresponds to copending U.S. patent application Ser. No. 119,042, filed on Nov. 10, 1987.
The polyimide is a novel high-temperature stable resin having many good properties. The polyimide exhibits excellent flowability and good processability. The fused resin, however, gradually exhibits decreased flowability which has an adverse effect on processability when the polyimide is kept at high temperatures for a long period of time, for example, long residence at high temperatures in a cylinder for injection molding.
Therefore, it is desirable to develop a polyimide which exhibits good flowability at lower temperatures and stable flowability for a long period of time during processing.
Previously produced molded products prepared by using composite materials composed of polyimides and fibrous reinforcing materials exhibit excellent mechanical strength, particularly strength retention at high temperatures and are also outstanding in solvent resistance and dimensional stability. Such molded products are desirable for use as structural members for spacecrafts and the like.
Polyimides generally have a high melt viscosity. Therefore composite materials containing these polyimides as matrices have required severe processing conditions as compared to those containing matrices of engineering plastics such as polycarbonate and polyethylene terephthalate, thereby causing various problems.
Special polyimides having low melt viscosity and excellent workability have also been known in the art. Such polyimides, however, have a low heat distortion temperature and are soluble in solvents such as halogenated hydrocarbons. Consequently, the composites containing such a resin as a matrix have caused problems in high-temperature stability and chemical resistance.
In order to overcome these problems, a composite material containing a polyimide having the above stated good properties and fibrous reinforcing materials has been developed, Koba et al; Japanese Laid-Open Patent TOKKAISHO 62-248635 (1987). The polyimide, however, gradually decreases in melt flowability when maintained at high temperatures for a long period. This phenomenon often inhibits continuous operation. Accordingly, the development of polyimide composite materials which are capable of operating for a long period of time and also provide excellent properties is strongly desired.